1. Field of the Invention
The invention relates to an optical recording medium capable of recording data thereinto or reproducing data therefrom, and more particularly to an optical recording medium suitable for carrying out so-called "simultaneous recording and verification" where confirmation as to whether data is properly recorded is carried out concurrently with recording data into a medium. The invention also relates to a method of optically recording data into an optical recording medium and reproducing data therefrom.
2. Description of the Related Art
As one of optical disc mediums capable of recording data thereinto and reproducing data therefrom both by radiating laser beam spot thereto is known a phase-change type optical disc. The phase-change type optical disc is capable of carrying out over-recording by a single beam, which is difficult to carry out by a magneto-optical (MO) disc. An optical head system for the phase-change optical disc is simpler than that for the MO disc.
A method of carrying out simultaneous recording and verification in such a phase-change type recording medium has been suggested in Japanese Unexamined Patent Publications Nos. 60-145537, 63-183624, and 6-349067. According to these Publications, when pulse beams are radiated onto the phase-change type recording medium, an intensity of reflected beams is varied almost at the same with the radiation. The suggested methods make it possible to confirm whether data is properly recorded by detecting the variation in the reflected beams by means of either the reflected beams itself or RF signals.
Roughly speaking, data is recorded into a phase-change type optical disc medium by the following steps: radiating laser beams onto a non-recorded region, which is in crystal condition, to thereby heat a recording layer; a temperature of the recording layer being raised; melting of the recording layer; and cooling down the recording layer to thereby reduce the recording layer amorphous. Hence, non-recorded and recorded regions have different phase conditions, and as a result, non-recorded and recorded regions have different reflectivity. Accordingly, an intensity of reflected laser beams is varied in accordance with the phase condition. A phase-change type optical disc medium utilizes such variation in an intensity of reflected laser beams for recording data thereinto.
It is known that a recording layer becomes as illustrated in FIG. 1, when a recording layer is molten by a laser beam spot. Thus, all regions of a recording layer are not always molten in a beam spot, even if a laser beam is radiated onto the recording layer. Herein, the molten region 60 corresponds to a region in which data is recorded, and the non-molten region 62 corresponds to a region in which data is not recorded yet. As mentioned earlier, the molten region 60 becomes amorphous when cooled down, whereas the non-molten region 62 remains crystal. If a recording layer is not molten, the recording layer is not reduced amorphous. In the beam spot, reflectivity of the recorded or amorphous region 60 coexists together with reflectivity of the non-recorded or crystal region 62, and hence it would be quite difficult to clearly distinguish the recorded region 60 from the non-recorded region 62. As a result, it would be difficult to accurately carry out simultaneous recording and verification only based on variation in the reflectivity. Hence, verification has to be carried out again after data has been recorded into a phase-change type optical disc medium in the above mentioned conventional mediums. As a result, it is substantially impossible to carry out simultaneous recording and verification in the conventional mediums.
In addition, in the conventional mediums where the recorded region has to have reflectivity quite different from that of the non-recorded region, there would be generated a large fluctuation in RF signals in over-recording, resulting in unstable verification together with reduction in reliability.